Factors affecting porosity of porous tungsten
Porous tungsten and its components based on porous tungsten are widely used in aerospace, power electronics and metallurgical industry. For example: vaporizer of mercury ion rocket engine; launcher of ion engine; porous tungsten matrix permeating coolant to make rocket nozzle throat lining; cathode framework of high-power gas filled thyristor; matrix of electrical contact material prepared by impregnation method and molten rare earth metal filtration. The porosity of porous tungsten affects its performance and the performance of various components. If the porous tungsten matrix of w-cu80 high voltage contact material is prepared by impregnation method, the porosity of porous tungsten matrix should be 33% ~ 36% to ensure the qualified contact material 1. The porosity of porous tungsten is affected by many factors, such as powder size, shape, chemical composition, molding method, molding pressure, sintering temperature, time and atmosphere. The effect of sintering temperature and porosity on the porosity of tungsten powder is discussed.
Grain size also has a great relationship. Fig. 1 shows the relationship between porosity and powder particle size and compaction pressure. It can be seen that for the same particle size powder, the compactor porosity decreases with the increase of molding pressure. This is due to the irregular surface of the powder, friction between each other, and the particles bridge each other to form the arch bridge effect. When the pressure is applied, the arch bridge effect in the powder body is destroyed, and the powder particles will fill each other's pores and rearrange their positions The end body was displaced. With the increase of pressure, the particles deform from the initial point contact to the surface contact. When the pressure continues to increase, the powder is mainly brittle fracture except for a small amount of plastic deformation. Therefore, in the process of powder forming, with the increase of molding pressure, the compactness of compacts increases gradually and the porosity decreases.
The porosity of powder compacts is smaller than that of fine compacts, which is due to the different particle sizes of powders and the different compaction behaviors, The smaller the loose density is, the worse the fluidity is, and the larger the filling volume is in the die. In this way, during the pressing process, the movement distance of the die and the internal friction force between the powders will increase, and the pressure loss will increase, which will affect the uneven distribution of the green density, resulting in low green density and large porosity.
